TRENTON — U.S. Sen. Frank Lautenberg, a pugnacious and plain-spoken Democrat who has represented New Jersey for three decades, will not seek re-election to a sixth term next year.

“I am not announcing the end of anything. I am announcing the beginning of a two-year mission to pass new gun safety laws, protect children from toxic chemicals and create more opportunities for working families in New Jersey,” Lautenberg told The Star-Ledger. “While I may not be seeking re-election, there is plenty of work to do before the end of this term and I'm going to keep fighting as hard as ever for the people of New Jersey in the U.S. Senate.”

The decision by Lautenberg, 89, who has been giving mixed signals for months, marks the end of a political era in New Jersey and one that is likely to set off a fierce primary battle as Democrats, including Newark Mayor Cory Booker, jockey for the rare open seat.

Lautenberg, who was a successful businessman, has been one of the Senate's most strident liberal voices over an approximately 30-year career, parrying with Democrats and Republicans alike on a broad array of domestic issues.

Recently, he has been a leader in pushing to renew a lapsed ban on assault weapons and restricting the size of ammunition magazines. He has been one of the biggest backers of Amtrak, and has clashed with the tobacco industry over the years, spearheading the effort to ban smoking on domestic airplane flights . He also sponsored a bill to withhold federal funds from states that did not raise their drinking age to 21, and helped rewrite hazardous waste cleanup law.

Sen. Frank Lautenberg will not seek re-electionOn the day news breaks that US Sen. Frank Lautenberg will not seek re-election, Star-Ledger editorial page editor Tom Moran analyzes whether Newark Mayor Cory Booker will cruise to the nomination to fill his seat and looks back at Lautenberg's career. Video by Mike Roy & Brian Donohue/The Star-Ledger

At home, he has tangled with Gov. Chris Christie on a number of issues, criticizing the Republican governor for slashing funds for the poor, nixing a new rail tunnel linking New Jersey and New York and merging Rutgers-Camden with Rowan University. And he has been critical of Democrats who have formed their own cozy relationships with Christie, accusing them of weakening the party.

Lautenberg is expected to make a formal announcement on at an event on Friday in Paterson.

Lautenberg, a World War II veteran, was born in Paterson to Russian and Polish parents. After the war, he worked as an insurance salesman before co-founding the firm Automatic Data Processing, which grew into a major corporation and made him a millionaire.

A rugged campaigner whom a Republican opponent once labeled “swamp dog,” Lautenberg won his first term in the Senate in 1982, after narrowly defeating Republican U.S. Rep. Millicent Fenwick. He won two more terms before retiring in 2000, a decision he immediately regretted.

When U.S. Sen. Robert Torricelli (D-N.J.) found himself in ethical trouble two years later and had to drop his campaign for re-election, Democrats pulled a last -minute and controversial maneuver to put Lautenberg on the ballot in his place.

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Lautenberg beat back a primary challenge from Rep. Rob Andrews (D-1st Dist.) in 2008 and easily won re-election in the general election. But he had to fight a battle of a different sort in 2010, when he learned he had non-Hodgkin Lymphoma. Lautenberg went through chemotherapy, during which some members of his staff shaved their heads in solidarity. He finished treatment in June 2010 and has been cancer free since then.

Next year, with Booker all but declaring his own candidacy for Senate, appeared to be shaping up differently than 2008. Several polls showed Lautenberg trailing Booker in the primary.

But the senator’s aides privately bristled that Booker had stepped out of line by so openly announcing his intentions, and peeved that he had not waited for Lautenberg to make a decision about his retirement first. And despite the polls, political experts weren’t counting Lautenberg out if he chose to run again.

While Booker is the leading Democratic candidate for Lautenberg’s seat, he is far from the only one. Rep. Frank Pallone (D-6th Dist.) has privately told party leaders he intends to run if Lautenberg retires. Assembly Speaker Sheila Oliver (D-Essex) has also expressed interest. In addition, Senate President Stephen Sweeney (D-Gloucester) has set up an exploratory committee.